Campylobacter infections have been increasingly recognized as major causes of human illness. Campylobacter fetus has been identified as a cause of bacteremia, and disseminated infections particularly in immunocompromised hosts, such as patients with malignancies and AIDS, or neonates. In ungulates, the natural hosts to this organism, C. fetus causes chronic mucosal infection, especially of the genito-urinary tract, and may be transmitted venereally. We have identified a family of surface array proteins (SAP) that encapsulate most, if not all, wild type C. fetus cells. Presence of these regular, paracrystalline SAPs renders C. fetus serum- and phagocytosis-resistant and bacteremic mice, whereas SAP(-) spontaneous mutants have these properties. Further, C. fetus can change the size of its predominant SAP. Such changes are associated with changes in antigenicity and crystalline structure. Two serotypes (A and B) of C., fetus have been recognized, based on lipopolysaccharide (LPS) characteristics. SAP from type A and B cells differ in N-terminus, pI, and specificity of binding to LPS. Attachment of SAP to the cell-surface is dependent on divalent cations. These studies of C. fetus have uncovered a variety of phenotypic characteristics. Of particular interest is the phenomenon of antigenic variation. The aims of this study are to understand the structural basis of the various properties identified (LPS- binding, cation-binding, complement interaction, antigenic sites, protein export functions) and to understand the basis for variation in SAP production. We will use genetic approaches to these problems. First, we plan to create isogeneic strains with defined mutations in the sapA locus. We will use miniTn3 mutagenesis of the cloned sapA gene in E. coli with transfer back into C. fetus to create a family of defined mutants, and then assay these strains for their phenotypic properties. Alternative approaches consist of creating chimeras of the A and B proteins, cloning the putative sapB gene from a type B strain, creating peptides using PCR, to inhibit specific functions, and by site-directed mutagenesis of identified points. To study regulation, we plan to clone sapA homologs that are silent to map their promoter regions and regions for potential recombination. Mutants of upstream regulatory loci will be made and assayed for expression of SAP phenotype. Finally, we plan to clone the recA gene, and then create an isogeneic recA(-) mutant to study the role of homologous recombination in antigenic variability.